First, don't purchase anything from CTD. BITOG's profanity filter would break if I typed what I really thought - let's just say they're incredibly unethical. Brownells, DSG, AIM Surplus, and MidwayUSA are much better choices for shooting supplies.
Second, look for a factory Remington piece or one from Nordic Components. The Nordic stuff isn't the cheapest, but it is extremely well made, right here in the USA.
Precise machining is paramount on a mag extension, so don't cheap out. If the extension doesn't mate properly to the factory mag tube, a ridge or dip can exist which can cause the follower or spring to bind, which can result in failures to feed. Rule of thumb for mag springs is 12" past the edge of the complete tube.